Abstract

The spontaneous tonic discharge activity of nigral dopamine neurons plays a fundamental role in dopaminergic signaling. To investigate the role of neuronal morphology and architecture with respect to spontaneous activity in this population, we visualized the 3D structure of the axon initial segment (AIS) along with the entire somatodendritic domain of adult male mouse dopaminergic neurons, previously recorded in vivo. We observed a positive correlation of the firing rate with both proximity and size of the AIS. Computational modeling showed that the size of the AIS, but not its position within the somatodendritic domain, is the major causal determinant of the tonic firing rate in the intact model, by virtue of the higher intrinsic frequency of the isolated AIS. Further mechanistic analysis of the relationship between neuronal morphology and firing rate showed that dopaminergic neurons function as a coupled oscillator whose frequency of discharge results from a compromise between AIS and somatodendritic oscillators. Thus morphology plays a critical role in setting the basal tonic firing rate, which in turn could control striatal dopaminergic signaling that mediates motivation and movement.

SIGNIFICANCE STATEMENT

The frequency at which nigral dopamine neurons discharge action potentials sets baseline dopamine levels in the brain, which enables activity in motor, cognitive, and motivational systems. Here we demonstrate that the size of the axon initial segment, a subcellular compartment responsible for initiating action potentials, is a key determinant of the firing rate in these neurons. The axon initial segment and all the molecular components that underlie its critical function may provide a novel target for the regulation of dopamine levels in the brain.

Footnotes

The authors declare no competing financial interests.

We are thankful to Cristian Gonzalez-Cabrera, Macarena Faunes, Andrea Riveros and Alejandro Oñate for technical assistance and to J. Paul Bolam for reading of the manuscript. We are especially thankful to Katia Gysling and Maria Estela Andrés' lab, DIDEMUC and VRI from Universidad Católica, and other colleagues and labs for their support following the fire that damaged our laboratory in 2015. This work was supported by Fondecyt N° 1141170 and Anillo ACT-1109 grants to P.H., NIH R01DA041705 to C.C., and a Conicyt fellowship to R.M.

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